Field of the Invention
The present invention lies in the field of semiconductor technology and, more specifically, relates to a method for fabricating a capacitor.
In order to further increase the integration level in semiconductor modules, materials having a high dielectric constant (high-.epsilon.)or having ferroelectric properties will be used to an increased extent in the future. By using these materials, it is hoped that a higher storage density (charge/area) will be gained in comparison with the capacitor dielectrics (oxide/nitride/oxide) used heretofore. A preferred material having a high dielectric constant is barium strontium titanate (BST) for example. Examples of ferroelectric materials that ought to be mentioned in the context are strontium bismuth tantalate (SBT) and also lead zirconium titanate (PZT).
However, the process integration of these materials entails a series of problems. Particular difficulties are posed by the deposition. The materials used as capacitor dielectric (metal oxide layers in ceramic form) can, for example, be applied conformally to a semiconductor surface by means of CVD deposition processes. The metal oxide layers are thereby applied inter alia directly to a first electrode of a storage capacitor. The material used for that electrode must be resistant to oxygen, since the metal oxide layer is deposited in an oxidizing atmosphere. Platinum and ruthenium, among others, have proved to be suitable electrode materials. One disadvantage of these materials, however, is that they exhibit a high level of diffusion into silicon or form metal silicide layers with silicon. Those layers can have an irreversible influence on the method of operation of active components.
Therefore, it is usual for a so-called barrier layer to be arranged underneath the electrode materials. The barrier layer prevents the electrode materials from diffusing into the silicon situated underneath. Barrier layers of this type are composed of a titanium/titanium nitride combination, for example.
It is unfavorable, however, that these barrier layers are sensitive to oxidation and can easily oxidize in the course of the CVD deposition of the metal oxide layers. Particularly when titanium is used as a constituent part of the barrier layer, there is the risk, therefore, that the usually conductive barrier layer will become insulating as a result of the oxidation. It is thus no longer possible to make contact with the first (lower) electrode of the storage capacitor.
In order to prevent the oxidation of the barrier layer during the CVD deposition, U.S. Pat. Nos. 5,883,781 and 5,943,547 (European patent application EP 0 739 030 A2), for example, propose the formation of lateral insulation webs around the patterned barrier layer. The insulation webs, which are composed of silicon nitride, are formed after the common etching of the lower electrode and also of the barrier layer by means of conformal deposition of a silicon nitride layer and anisotropic etching thereof. The silicon nitride layer thereby essentially remains in the lower region of the electrode, that is to say only in the region of the barrier layer.
Lateral insulation webs around the barrier layer are also proposed in U.S. Pat. No. 5,656,852. The insulation webs, which may be composed of silicon dioxide or silicon nitride are formed there prior to the deposition of the lower metal electrode.
A common feature of all of the prior art insulation webs is that they are formed from a conformally deposited insulation layer with subsequent anisotropic etching. The necessary etching step, in particular, may thereby attack the barrier layer and/or the electrode or one of these two layers may be contaminated by the insulation layer.
The contamination of the metal electrode, in particular, is disadvantageous since the material properties of the metal oxide layer to be applied to the metal electrode depend to a great extent on the quality of the interface between metal electrode and metal oxide layer.
By contrast, insulation webs formed in situ are disclosed in U.S. Pat. No. 5,554,844, in which a titanium nitride layer serving as a barrier is oxidized in its side regions prior to the deposition of the capacitor dielectric. Since the oxidation of the barrier layer progresses rapidly in particular with respect to the adjoining metal electrode, problems concerning the adhesion of the metal electrode on the barrier may arise.